1. Field of the Invention
This invention relates to a method for gasketing a fuel cell stack in which the gasket materials are applied to the bi-polar separator plates of the fuel cell stacks using a screen printing process. More particularly, this invention relates to a method for gasketing polymer electrolyte membrane (PEM) fuel cell stacks in which the gasket materials are applied to the bipolar separator plates thereof, which plates are made from graphite/carbon compositions. The material used to produce the gaskets is silicone. The method enables application of a fixed thickness of silicone of a desired Durometer to the separator plate for sealing whereby, in addition to fluid sealing, a compressive force is achieved in the active zone so as to maintain proper electrical conduction within the fuel cell stack.
2. Description of Prior Art
Generally speaking, fuel cell electrical output units are comprised of a stacked multiplicity of individual cells separated by bi-polar electronically conductive separator plates. Individual cells are sandwiched together and secured into a single staged unit to achieve desired fuel cell energy output. Each individual fuel cell generally includes an anode and cathode electrode, a common electrolyte, and a fuel and oxidant gas source. Both fuel and oxidant gases are introduced through manifolds, either internal or external to the fuel cell stack, to the respective reactant chambers between the separator plate and the electrolyte.
There are a number of fuel cell systems currently in existence and/or under development which are designed for use in a variety of applications including power generation, automobiles, and other applications where environmental pollution is to be avoided. These include molten carbonate fuel cells, solid oxide fuel cells, phosphoric acid fuel cells, and polymer electrolyte membrane fuel cells (also referred to as proton exchange membrane fuel cells).
Commercially viable fuel cell stacks may contain up to about 600 individual fuel cell units, each having a planar area up to twelve square feet. In stacking such individual cells, separator plates separate the individual cells, with fuel and oxidant each being introduced between a set of separator plates, the fuel being introduced between one face of the separator plate and the anode side of an electrolyte and oxidant being introduced between the other face of the separator plate and the cathode side of a second electrolyte. Cell stacks containing 600 cells can be up to twenty feet tall, presenting serious problems with respect to maintaining cell integrity during heat-up and operation of the fuel cell stack. Due to thermal gradients between cell assembly and cell operating conditions, different thermal expansions, and the necessary strength of materials required for the various components, close tolerances and very difficult engineering problems are presented. In this regard, cell temperature control is highly significant and, if it is not accomplished with a minimum temperature gradient, uniform current density will not be maintainable, and degradation of the cell will occur.
In a polymer electrolyte membrane fuel cell, the electrolyte is an organic polymer in the form of a proton conducting membrane, such as a perfluorosulfonic acid polymer. The separator plates of a polymer exchange membrane fuel cell stack are used to channel air to the cathode sides, hydrogen-rich gas to the anode sides, and a cooling medium between the anode and cathode. Gaskets are required to prevent these gases and liquids from directly contacting each other within the stack or from leaking out across perimeter seals. Providing an effective seal is particularly problematic due to the nature of the graphite/carbon bi-polar separator plates utilized in polymer electrolyte membrane fuel cell stacks. More particularly, it is known that many materials do not bond or otherwise adhere well to graphite/carbon components, particularly under fuel cell operating conditions.
One solution is to apply the sealing material to other components of the fuel cell stack. U.S. Pat. No. 6,057,054 to Barton et al. teaches a membrane electrode assembly for electrochemical fuel cell comprising coextensive ion exchange membrane and electrode layers and a resilient fluid impermeable integral seal made by impregnating a sealing material into the porous electrode layers in the sealing regions. The uncured sealant material is a flow processable elastomer that is applied to the membrane electrode assembly using a vacuum injection molding process. The elastomer material is selected from the group consisting of silicones, fluorosilicones, fluoroelastomers, ethylene propylene di-methyl, and natural rubber.
U.S. Pat. No. 4,588,661 to Kaufman et al. teaches a bi-polar gas reactant distribution assembly for use in a fuel cell having a solid edge seal comprising a solid, fusible, gas impervious edge sealing compound produced from thermoplastic resins or polyethersulfone.
U.S. Pat. No. 5,096,786 to Granata, Jr. et al. teaches a phosphoric acid fuel cell having integral edge seals formed by an elastomer permeating an outer peripheral band contiguous with the outer peripheral edges of the cathode and anode assemblies and the matrix. The elastomer is a copolymer of tetrafluoroethylene and propylene.
Screen-printing as a means for applying a flowable material onto a substrate is used extensively in the ink printing business. This same technology has been extended to several industries such as manufacturers of automotive gaskets. Such gaskets have proven to be a very cost-effective method to produce large quantities of gaskets with silicone or similar heat curable sealing materials with thicknesses of less than 10 mils. U.S. Pat. No. 4,001,042 to Trocciola et al. teaches a method for producing a fuel cell electrode/matrix element in which a hydrophilic electrolyte matrix layer is screen-printed onto the surface of a fuel cell gas diffusion electrode, which electrode includes a catalyst layer on the surface onto which the matrix layer is printed. The step of printing includes forming an ink by mixing an aqueous solution of polyethylene oxide with a matrix material and printing to the surface of the electrode through a screen. Thereafter, the matrix layer is heat treated to remove most of the aqueous solution of polyethylene oxide.
It is one object of this invention to provide a method for sealing fuel cell separator plates comprising graphite/carbon compositions employed in fuel cell stacks.
It is another object of this invention to provide a graphite/carbon bi-polar separator plate suitable for use in polymer electrolyte membrane fuel cells and comprising integral seal means.
It is yet another object of this invention to provide a polymer electrolyte membrane fuel cell stack comprising graphite/carbon bi-polar separator plates having integral seals.
These and other objects of this invention are addressed by a fuel cell stack comprising a plurality of polymer electrolyte membrane fuel cell units having an anode electrode, a cathode electrode and a polymer electrolyte membrane disposed between the anode electrode and the cathode electrode. A graphite/carbon bi-polar separator plate having a centrally disposed active region substantially coextensive with the electrodes separates each of the fuel cell units between the anode electrode and the cathode electrode of adjacent fuel cell units. The graphite/carbon bi-polar separator plates comprise distribution means for providing an oxidant to the cathode electrode, a fuel gas to the anode electrode and a cooling medium between the anode electrodes and the cathode electrodes. Typically, the separator plates form internal conduits through which the cooling medium is flowing. The graphite/carbon bi-polar separator plates of this invention are provided with substantially flat peripheral sealing regions on each face extending around the centrally disposed active region. The fuel cell stack further comprises gasket means for preventing the oxidant, the fuel gas and the cooling medium from mixing. The gasket means comprises a gasketing material comprising silicone disposed on the substantially flat peripheral sealing region on each face of the bi-polar separator plate, which gasket material contacts the polymer electrolyte membrane, forming a peripheral silicone seal. The gasket material is applied to the substantially flat peripheral sealing regions on each face of the graphite/carbon bi-polar separator plates by a screen-printing process, the details of which are discussed hereinbelow.
The method for integral sealing of a fuel cell stack in accordance with this invention comprises the steps of forming a graphite/carbon composition into a graphite/carbon separator plate having a centrally disposed active region and a substantially flat peripheral sealing region surrounding the centrally disposed active region. A silicone gasket material is applied onto the substantially flat peripheral sealing region completely around the centrally disposed active region, forming a silicone gasket. The graphite/carbon separator plate comprising the silicone gasket is inserted between adjacent fuel cell units of the fuel cell stack, whereby the silicone gasket contacts a peripheral region of a polymer electrolyte membrane of the adjacent fuel cell units, forming a peripheral seal between the graphite/carbon separator plate and the polymer electrolyte membranes.